The intermetallic compound triniobium tin, Nb.sub.3 Sn, is a type-II metallic superconductor of interest because it has high values of superconducting critical current density in high magnetic fields. Critical current density, J.sub.c, is a value resulting from division of the critical current measured in a magnetic field by the cross-sectional area of the superconductor.
In type-II superconductors, the critical current density J.sub.c is controlled by microstructural heterogeneities which pin the fluxoid lattice, with strong pinning leading to high transport currents. In triniobium tin, it has been suggested that grain boundaries are the primary flux-pinning centers, and thus the control of grain size is essential to the superconducting properties of this material. A relation is known to exist between the superconductor grain size and critical current density: finer grain sizes lead to higher critical current densities.
Historically, triniobium tin has been formed by a number of different processes. These include: condensation from the vapor phase; crystallization from the liquid phase; diffusion between one solid phase and one liquid phase; and diffusion between two solid phases. The solid-liquid diffusion method for making triniobium tin superconductor comprises passing a niobium-based substrate through a bath of molten tin or tin alloy and then heat-treating it to form a layer of triniobium tin on the surface. However, the critical current density of triniobium tin produced by the aforesaid conventional method abruptly decreases in a high magnetic field.
In recent years, superconducting wires made of triniobium tin for large size magnets have been required to possess a higher critical current within a strong magnetic field. To obtain a magnet capable of generating a high magnetic field, it is necessary to use superconducting wires having both a high upper critical magnetic field and a high critical current density.
Efforts have been made to improve the critical current density of superconducting triniobium tin wire by ternary or quaternary additions to the bronze matrix or to the niobium core wires. Caslaw, British Patent No. 1,342,726, found that up to forty-five percent copper in the tin bath improved the critical current density and the reaction kinetics during the formation of triniobium tin tape. Other improvements in critical current density of triniobium tin have been found by adding gallium, indium, silver, and aluminum to the tin bath, and adding hafnium and titanium to the niobium core. This is the subject of Tachikawa U.S. Pat. No. 4,323,402.
There is a need to provide a triniobium tin superconductor which has a high critical current density and gives a magnet capable of generating a strong magnetic field.
There is also a need for a process that produces triniobium tin superconductors with high critical current density.